Organic polymers containing at least one reactive silyl group in the molecule are known to have properties such that they are cross-linkable by siloxane bond formation accompanied by reactions such as hydrolysis of a reactive silyl group due to moisture and the like, even at room temperature, to provide rubber-like cured products.
Among these reactive silyl group-containing polymers, those polymers which have a main chain skeleton of a polyoxyalkylene polymer or a polyisobutylene polymer are disclosed in Patent Documents 1 and 2, and the like. These polymers have already been industrially produced and used in various applications such as sealants, adhesives, and coatings.
Curable compositions containing these reactive silyl group-containing organic polymers are cured with use of a silanol condensation catalyst. Common examples of the silanol condensation catalyst include organotin catalysts having a carbon-tin bond such as dibutyltin bis(acetylacetonate). In recent years, however, organotin compounds are known to be toxic, and the development of non-organotin catalysts has been desired. Patent Documents 3, 4, 5, 6, and 7 disclose tin carboxylates and other carboxylate metal salts as silanol condensation catalysts. These documents also disclose that addition of an amine compound as a promoter improves curability. In consideration of the environmental load, catalysts containing substantially no metal have also been desired, and Patent Document 8 discloses that combination use of an amine compound and a carboxylic acid provides a metal-free silanol condensation catalyst.
As disclosed in Non-Patent Document 1, phosphorus compounds containing phosphorus having an oxidation number of +3 are widely used as stabilizers for organic materials and flame retardants. However, it has not been known that these compounds suppress crystallization which occurs when curable compositions including a silyl group-containing polymer are cured. Patent Document 9 discloses an antibacterial sealant composition prepared with use of a phosphorus compound having a specific structure. However, it has not been predictable that the compound suppresses crystallization mentioned above.
Meanwhile, phosphorus plasticizers such as tricresyl phosphate are used for softening polyvinyl chloride and for providing sealants with flame retardance. In consideration of their properties, those plasticizers which are in a liquid form at room temperature, namely, ones having a melting point of lower than 23° C. are commonly used.    Patent Document 1: Japanese Kokai Publication S52-73998    Patent Document 2: Japanese Kokai Publication S63-6041    Patent Document 3: Japanese Kokai Publication H05-39428    Patent Document 4: Japanese Kokai Publication H09-12860    Patent Document 5: Japanese Kokai Publication 2000-313814    Patent Document 6: Japanese Kokai Publication 2000-345054    Patent Document 7: Japanese Kokai Publication 2003-206410    Patent Document 8: Japanese Kokai Publication H05-117519    Patent Document 9: Japanese Kokai Publication H10-251616    Non-Patent Document 1: Akisada Endo and Makoto Sudo, “Plastics Haigouzai: Kiso to Ouyou”, Taiseisha Ltd., Nov. 30, 1996, pp. 90-91